1. Field of the Invention
The present invention relates to dielectric filters to be incorporated into mobile communications devices for determining the frequency band pass characteristics of signals to be processed.
2. Description of the Related Art
With mobile communications devices such as portable telephones adapted to have a multiplicity of functions in recent years, it is required that these devices be usable for a multiplicity of bands and have satisfactory frequency characteristics in wide frequency bands. It is known that dielectric filters are used in such mobile communications devices for transmitting and receiving signals, for example, in a frequency band of hundreds of megahertz to several gigahertz. FIG. 7 shows the construction of a dielectric filter for use in conventional mobile communications devices (see JP-B No. 7-254806).
With reference to FIG. 7, a dielectric block 1 in the form of a rectangular parallelepiped and made from a dielectric material such as a ceramic material has two through bores 2, 4 extending through the block from the front end face 14 thereof to the rear end face 12 thereof. The through bores 2, 4 comprise large-diameter bore portions 21, 41 and small-diameter bore portions 22, 42, respectively. An outer conductor 5 comprising a conductor layer of silver or the like is formed over the outer peripheral surface (upper and lower surfaces and opposite side surfaces) of the dielectric block 1. An inner conductor 3 is formed over each of the inner peripheral surfaces of the block defining the through bores 2, 4. A conductor layer is formed also on the rear end face 12 of the block 1 to provide a short-circuiting end face for short-circuiting the outer conductor 5 and the inner conductor 3. The dielectric block 1 is left exposed over the front end face 14 thereof to provide an open end face to leave the through bore 2 and the inner conductor 3 open.
A pair of input and output electrodes 6, 6 each comprising a conductor layer are formed on the outer peripheral surface of the block 1. Electrode separating regions 8, 8 are formed between the outer conductor 5 and the input and output electrodes 6, 6 by cutting out the conductor layer. A bridge portion 51 comprising a part of the outer conductor 5 is formed between the input and output electrodes 6, 6.
FIG. 9 is an equivalent circuit diagram of the conventional dielectric filter described. A pair of resonators A, A are coupled to the input and output electrodes 6, 6, respectively, each via a capacitance C1, and the two resonators A, A are coupled to each other by a magnetic field M.
FIG. 8 shows the frequency characteristics of the conventional dielectric filter described. There is an attenuation pole c1 at the high frequency side of a pass band a1, and there is an attenuation pole b1 at the low frequency side thereof, whereby a band pass filter is realized.
With the dielectric filter shown in FIG. 7, a resonance frequency is set in the desired pass band a1 by varying the axial lengths of the small-diameter bore portions 22, 42 of the through bores 2, 4, and the width of the pass band is adjustable by varying the distances of the input and output electrodes 6, 6 from the large-diameter bore portions 21, 41 of the through bores 2, 4 and thereby varying the degree of the capacitance coupling.
With the conventional dielectric filter shown in FIG. 7, the position (frequency) of the attenuation pole b1 at the low frequency side is adjustable to some extent by altering the external dimensions of the dielectric block and the arrangement of the large-diameter bore portions and the small-diameter bore portions of the through bores, whereas difficulty is encountered in making a similar adjustment of the attenuation pole c1 at the high frequency side. Moreover, the fabrication of the filter involves the problem that it is very cumbersome to alter the shape of the through bores of the dielectric block.
Although the attenuation characteristics in a high frequency band outside the pass band also become important because communications devices in recent years are adapted to have a multiplicity of functions, conventional dielectric filters have the problem of being unable to fully attenuate a higher harmonic e1 produced in a high frequency band (around 5 GHz) corresponding to about 3 times the pass band a1 as shown in FIG. 8 and being seriously impaired in filter characteristics.